The present invention relates to plasma generators, and more particularly, to a method and apparatus for generating a plasma to sputter deposit a layer of material or to etch a layer of material in the fabrication of semiconductor devices.
Low pressure plasmas have become convenient sources of energetic ions and activated atoms which can be employed in a variety of semiconductor device fabrication processes including surface treatments, depositions, and etching processes. For example, to deposit materials onto a semiconductor wafer using a sputter deposition process, a plasma is produced in the vicinity of a sputter target material which is negatively biased. Ions created adjacent the target impact the surface of the target to dislodge, i.e., xe2x80x9csputterxe2x80x9d material from the target. The sputtered materials are then transported and deposited on the surface of the semiconductor wafer.
Sputtered material has a tendency to travel in straight line paths, from the target to the substrate, being deposited, at angles which are oblique to the surface of the substrate. As a consequence, materials deposited in etched openings, including trenches and holes of semiconductor devices having openings with a high depth to width aspect ratio, may not adequately coat the walls of the openings, particularly the bottom walls. If a large amount of material is being deposited, the deposited material can bridge over, causing undesirable cavities in the deposition layer. To prevent such cavities, sputtered material can be redirected into substantially vertical paths between the target and the substrate by negatively biasing (or self-biasing) the substrate and positioning appropriate vertically oriented electric fields adjacent the substrate if the sputtered material is sufficiently ionized by the plasma. However, material sputtered by a low density plasma often has an ionization degree of less than 10% which is usually insufficient to avoid the formation of an excessive number of cavities. Accordingly, it is desirable to increase the density of the plasma to increase the ionization rate of the sputtered material in order to decrease the formation of unwanted cavities in the deposition layer. As used herein, the term xe2x80x9cdense plasmaxe2x80x9d is intended to refer to one that has a high electron and ion density, in the range of 1011-1013 ions/cm3.
There are several known techniques for exciting a plasma with RF fields including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through a coil surrounding the plasma induces electromagnetic currents in the plasma. These currents heat the conducting plasma by ohmic heating, so that it is sustained in a steady state. As shown in U.S. Pat. No. 4,362,632, for example, current through a coil is supplied by an RF generator coupled to the coil through an impedance-matching network, such that the coil acts as the first windings of a transformer. The plasma acts as a single turn second winding of a transformer.
Although ionizing the deposition material facilitates deposition of material into high aspect ratio channels and vias, many sputtered contact metals have a tendency to deposit more thickly in the center of the wafer as compared to the edges. This xe2x80x9ccenter thickxe2x80x9d deposition profile is undesirable in many applications where a uniform deposition thickness is needed.
As described in copending application Ser. No. 08/680,335, entitled xe2x80x9cCoils for Generating a Plasma and for Sputtering,xe2x80x9d filed Jul. 10, 1996 (Attorney Docket # 1390CIP/PVD/DV) and assigned to the assignee of the present application, which application is incorporated herein by reference in its entirety, it has been recognized that the coil itself may provide a source of sputtered material to supplement the deposition material sputtered from the primary target of the chamber. Application of an RF signal to the coil can cause the coil to develop a negative bias which will attract positive ions which can impact the coil causing material to be sputtered from the coil. Because the material sputtered from the coil tends to deposit more thickly at the periphery of the wafer, the center thick tendency for material sputtered from the primary target can be compensated by the edge thick tendency for material sputtered from the coil. As a result, uniformity can be improved.
It has been recognized by the present applicant that the sputtering rate for material sputtered from the coil may be nonuniform around the perimeter of the coil. Hence the ability to achieve a desired level of uniformity may be adversely affected in some applications.
It has further been recognized by the present applicant that the coil can develop a hot spot which can cause uneven heating of the substrate. This uneven heating of the coil can also cause reliability problems in that portions of the coil may become too hot and deform, and may also cause particulates deposited on the coil to flake off and contaminate the substrate. Since single turn coils are typically required to carry a relatively high level of current, these problems can be more pronounced in such single turn coils.
It is an object of the present invention to provide a method and apparatus for etching or sputter depositing a layer which improves uniformity and which obviates, for practical purposes, the above-mentioned limitations.
These and other objects and advantages are achieved by a plasma generating apparatus in which, in accordance with one aspect of the invention, an impedance-circuit coupled to an RF coil has a tunable variable reactance for shifting RF voltage distributions along the length of the RF coil. It has been noted that the RF voltage distributions along the coil influence the plasma properties such as the plasma density and potential profiles and electron and ion movements including ion bombardment of the coil and substrate being deposited. It has further been noted that the instantaneous RF voltage distributions along the coil are not uniform or symmetric about the symmetry axis of the coil. These nonuniform and asymmetrical instantaneous RF voltage distributions along the coil can lead to nonuniform and asymmetrical heating of both the coil and the substrate as well as nonuniform sputtering of the coil and nonuniform deposition of material on the substrate.
In accordance with one aspect of the present invention, it has been found that the reactance between the RF coil and the ground can be cyclicly and continuously tuned during a sputtering operation to move or vary the RF voltage distributions along the RF coil so that minima and maxima points of the RF voltage distribution along the coil are not fixed at particular regions of the coil. Instead, the RF voltage distribution can be repeatedly moved around the coil in a rotational or other motion. In addition, the ionization pattern of the plasma associated with the RF voltage distribution may be similarly moved in conjunction with the movement of the RF voltage distribution. As a consequence, the RF coil and substrate can be more uniformly and symmetrically heated, by time-averaging, because a xe2x80x9chot spotxe2x80x9d of sputtering can be avoided. In addition, the coil itself may be more uniformly sputtered and the deposition material can be more uniformly deposited.
In another aspect of the present invention, the reactance of the tunable variable reactance can be repeatedly changed during the deposition to shift the voltage distributions along the coil, without requiring corresponding impedance rematches as a result of the impedance changes. In many applications, it is desirable to match the impedance of the coil and associated impedance circuitry to the impedance of the RF generator so as to minimize the reflection of RF energy back to the generator. Here, the voltage distributions can be rotated during the deposition without having to rematch impedances because the combined impedances of the coil and the impedance network can remain substantially constant, even though the reactance of the tunable variable reactance is repeatedly changed during the deposition.
In one embodiment, the tunable variable reactance includes a pair of tunable inductors and a pair of linked core pieces movably disposed within the pair of tunable inductors in which one of the pair of tunable inductors is positioned between the RF coil and the ground. As explained in greater detail below, the core pieces compensate each change in the inductive reactance of one tunable inductor of the pair with a corresponding substantially equal but opposite change in the inductive reactance of the other tunable inductor of the pair so as to keep the sum of the inductive reactances of the tunable inductors of the pair substantially constant. As a result, the need to rematch the RF coil impedance, once the RF coil impedance has been adequately matched, can be reduced or eliminated.
In an alternative embodiment, the tunable variable reactance includes a pair of variable capacitors and a dielectric piece movably disposed within the pair of variable capacitors in which one of the pair of variable capacitors is positioned between the RF coil and the ground. The dielectric piece compensates a change in the capacitive reactance of one variable capacitor of the pair with a corresponding change in the capacitive reactance of the other variable capacitor of the pair so as to keep the sum of the capacitive reactances of the variable capacitors of the pair substantially constant.